1. Field of the Invention
The present invention relates to an IT-cut quartz crystal unit, and more particularly to an IT-cut crystal unit which can provide satisfactory vibration characteristics.
2. Background of the Invention
Quartz crystal units for use as reference sources of frequency and time are classified into several types of “cuts” according to the crystallographic orientation in which a vibrating plate, i.e., a quartz crystal blank, forming part of a crystal unit, is cut from a single crystal of quartz. Conventionally, X-cut, AT-cut, BT-cut, and the like have been known as such types of cuts. Also, an SC-cut crystal unit is known as a double-rotation crystal unit, for example, as disclosed in JP-A-11-177376. The double-rotation crystal unit cited here refers to a crystal unit which has a crystal blank that is cut from a crystal of quartz in the following manner. A plane perpendicular to one of three crystal axes of the quartz is rotated over a predetermined angle about one of the remaining two crystal axes, and is further rotated over another predetermined angle about the last crystal axis. The crystal blank of the double-rotation crystal unit is cut from the crystal of quartz along the thus rotated plane.
As Illustrated in FIG. 1, for example, an SC-cut crystal unit uses crystal blank 1 which is cut from a crystal of quartz along a plane perpendicular to the Y-axis of the crystal of the quartz which is rotated over approximately 30° about the X-axis, and further rotated over approximately 22° about the Z-axis from the rotated position. Excitation electrodes are formed on both main surfaces of this crystal unit 1.
Like the AT-cut crystal unit, the SC-cut crystal unit has the temperature-frequency characteristic which can be represented by a cubic function curve, and is used in a temperature region near an extremal value on the cubic function curve, where the frequency is deemed to vary less due to changes in temperature. The SC-cut crystal unit has an inflection point on the cubic function curve at 94° C., and is used as a crystal unit for an oven-controlled crystal oscillator because it has a inflection point at such a high temperature.
However, the SC-cut crystal unit has the inflection point at 94° C., so that if an extremal value at a temperature lower than the inflection point is set for a target temperature of the oven, the target temperature will be in the range of 65° C. to 85° C. Thus, if the SC-cut crystal unit is utilized in a crystal oscillator which is used over a wide temperature range, for example, when it is required to operate up to 80° C. on the high temperature side, a smaller margin between the set temperature of the oven and the ambient temperature will make it difficult to control the oven at a fixed temperature, thereby failing to maintain a fixed oscillation frequency.
To solve this problem, it is under review to use an IT-cut quartz crystal unit instead of the SC-cut crystal unit. The IT-cut crystal unit, which is a double-rotation crystal unit similar to the SC-cut crystal unit, utilizes crystal blank 2 which is cut from a crystal of quartz along a plane perpendicular to the Y-axis of the crystal of the quartz which is rotated over approximately 34° about the X-axis, and further rotated over approximately 19° about the Z-axis from the rotated position. Excitation electrodes are formed on both main surfaces of crystal blank 2.
While the IT-cut crystal unit has the temperature-frequency characteristic which exhibits a change similar to the temperature-frequency characteristic of the SC-cut crystal unit, its inflection point temperature is at 74° C. which is lower than that of the SC-cut crystal unit. In addition, the IT-cut crystal unit has an extremal point in the range of 85° C. to 105° C. on the higher temperature side of the inflection point. For this reason, a crystal oscillator using the IT-cut crystal unit can set an oven at a temperature in the range of 85° C. to 105° C., even if it is required to operate at a maximum temperature of 80° C. and can relatively readily control the temperature.
However, there are very few instances of IT-cut crystal units, and particularly, any solution has not been brought about for the relationship between a position at which the crystal blank is supported and the vibration characteristics.